Answer
In kJ/mol: -17kj/mol
In kcal/mol: -4kcal/mol
Work Step by Step
In the given reaction, bond breaking and bond formation takes place. Bond breaking is endothermic (positive) whereas bond formation is exothermic.
In order to find $\Delta$ $H^{\circ}$ of the reaction, we add the bond dissociation energies, keeping in mind that where the bond formation takes place, the bond dissociation energy is negative.
Therefore,
In kJ/mol
Bonds broken=
$CH3CH2-Cl=+339$
$H-I= +297$
Bonds formed=
$CH3CH2-I=-222$
$H-Cl=-431$
$\Delta$$H^{\circ}$= $(+339+297) + (-222) + (-431)$
$\Delta$$H^{\circ}$= $+636+(-653)$
$\Delta$$H^{\circ}$= $+636 -653$
$-17 kJ/mol$
In kcal/mol
Bonds broken=
$CH3CH2-Cl= +81$
$H-I= +71$
Bonds formed=
$CH3CH2-I=-53$
$H-Cl=-103$
$\Delta$$H^{\circ}$=$ (+81+71 )+ (-53)+(-103)$
$\Delta$$H^{\circ}$ =$152+ ( -156)$
$\Delta$$H^{\circ}$ = $152 - 156$
$\Delta$$H^{\circ}$= $-4 kcal/mol$